A lactose operator segment has been cloned, in up to 12 tandem, repeat copies in the pMB9 plasmid. This permits both the purification of the EcoR1 -excisable operator in milligram quantities and its characterization by physical-chemical techniques, as well as the study of repressor binding to isolated operator and polyoperator segments via filter-binding and other techniques. We will determine conformational and length parameters of the free and repressor-bound operator or polyoperator segments by circular dichroism and electron microscope studies. We will also study the effects of various mutations in the operator sequence on these parameters.